1. Field of the Invention
The present invention relates to an active matrix type liquid-crystal display unit which is so intended as to suppress a fluctuation in potential of a signal (data), thereby reducing a power consumption. Also, the present invention relates to a display method for an active matrix type liquid-crystal display unit using an in-plane switching mode which is also called "IPS".
2. Description of the Related Art
In a liquid-crystal display unit, the inversion of a voltage applied to a liquid-crystal element has been required. This operation is conducted to prevent the deterioration of display such as after-image phenomenon because the deterioration of material such as liquid crystal or orientation film, or parasitic charges by impurities are caused in case of applying an electric field having a single polarity for a long time. This operation is called "a.c. operation", and one inversion has been required for one frame (field) or several frames. For the operation, there have existed a variety of systems such as the inversion of a frame (field inversion) in which an entire display screen of one frame has the same polarity (FIG. 11A), a line inversion in which the polarity for one line is the same line but the polarity for a line is different from that for adjacent lines (FIGS. 11B and 11C), a dot inversion in which all pixels adjacent to each other are different in polarity from each other (FIG. 11D), and so on.
Up to now, in order to conduct the above inversion, a signal for inverting the polarity has been supplied to pixels from a data driver (signal driver). FIG. 8 shows a unit pixel for a conventional active matrix type liquid-crystal display unit in which a thin-film transistor (T) is controlled by a signal from a scanning line Xn, and a signal from a data line (Pm) is sent to a liquid-crystal element (LC), and an auxiliary capacity (C) which is disposed in parallel with the liquid-crystal element if required so that charges are stored in an on-state (FIG. 8).
A drive signal for a display unit in which the unit pixels of the above type are disposed in the form of a matrix is shown in FIG. 9. In the figure, CLK is a clock signal (synchronous signal) which represents a minimum time for the display unit. A signal is produced in accordance with CLK. Pulses are sequentially applied to scanning lines (X.sub.1, X.sub.2, X.sub.3, . . . X.sub.N-1, X.sub.N) as shown in the figure. Data corresponding to image signals for each line are applied to a data line P.sub.1. This shows an example of the field inversion (FIG. 11A). For comparison, image information is set to be always identical with each other. In other words, 2nd-field data is to invert 1st-field data with respect to an earth level. The same is applied to 2nd-field data and 3rd-field data. An example of data of the line inversion (FIG. 11C) is shown in FIG. 10. Comparing data corresponding to each line, the 1st field is inverse in polarity to the 2nd field.
The conventional liquid-crystal display unit conducts display by applying a voltage vertical to substrates between the substrates, whereas the above display unit conducts display by applying a voltage parallel to a substrate plane within a substrate. The drive system of this type is called "in-plane switching (IPS)". The fundamental concept in the case where the above system is applied to the active matrix type liquid-display unit using a thin-film transistor as a switching element is disclosed in Japanese Patent Examined Publication No. Sho 63-21907.
In addition, the application of the above system is also disclosed in Japanese Unexamined Patent Publication No. Hei 7-43744, Japanese Unexamined Patent Publication No. Hei 7-43716, Japanese Unexamined Patent Publication No. Hei 7-36058, Japanese Unexamined Patent Publication No. Hei 6-160878, Japanese Unexamined Patent Publication No. Hei 6-202073, Japanese Unexamined Patent Publication No. Hei 7-134301, and Japanese Unexamined Patent Publication No. Hei 6-214244. Further, the application of the above system to a simple matrix type liquid-crystal display unit is disclosed in Japanese Unexamined Patent Publication No. Hei 7-72491, and the application of the above system to an active matrix type liquid-crystal display unit having a thin-film diode as a switching element is disclosed in Japanese Unexamined Patent Publication No. Hei 7-120791.
The principle of the IPS system disclosed in the above publications will be described in brief with reference to FIG. 6 and FIG. 7. FIG. 6 shows a unit pixel for the active matrix type liquid-crystal display unit using the IPS system. As in the normal active matrix type liquid-crystal display unit, a plurality of data lines 1 and a plurality of scanning lines 2 are disposed in the form of a matrix. In addition, a plurality of earth lines 3 (earth line or opposite electrode line) are disposed. In the conventional display unit, because electrodes for an opposite substrate are disposed, no earth lines 3 are required. On the other hand, since there are disposed no electrodes for the opposite substrate in the IPS system, there is required the provision of wiring having the same function as that of such electrodes for the opposite substrate.
The earth lines 3 are normally held constant in potential. Also, because the earth lines 3 are formed together with the scanning lines 2, the former does not intersect with the latter, that is, a parallel structure is provided. This is because a part of the earth lines 3 is partially overlapped to a part of pixel electrodes 4 which are formed together with the data lines 1 in such a manner that auxiliary capacities (C) are formed. In other words, the scanning lines 2 and the earth lines 3 are formed simultaneously, and the data lines 1 and the pixel electrodes 4 are formed simultaneously. TFTs 5 each having a part of the scanning line 2 as a gate electrode are formed as shown in the figure. A source of each TFT 5 is in contact with the date line 1, and a drain thereof is in contact with the pixel electrode 4 (FIG. 6).
With such a structure that the earth lines 3 are disposed to be opposite to the pixel electrode 4, an electric field is developed between the pixel electrode 4 and the earth line 3 as indicated by arrows. Liquid-crystal molecules are, as indicated by a in FIG. 7, initially oriented with a given angle, for example, 45.degree. with respect to an intended electric field. Then, upon the application of an electric field, the liquid-crystal molecules are intended to be in parallel to the electric field, as indicated by b in FIG. 7. Well using the inclination of the liquid-crystal molecules, variable density can be expressed. The above is the principle of the IPS system (FIG. 7).
As described above, the conventional active matrix type liquid-crystal display unit requires that data having variation twice as much as the variation of a signal required by only image information is produced by a driver. In other words, although there is merely required that an effective voltage of 5 V is applied to liquid crystal, a drive capability in a range of 10 V which is from +5 V to -5 V has been required because of the necessity of inversion. This leads to the largest obstruction to a reduction of the drive voltage of the driver and a reduction of power consumption.
Likewise, the above display unit suffers from such problems as the destroy of a transistor and the deterioration of characteristics, which are caused by applying an excessive voltage to the active matrix circuit.